The present invention relates to a method for operating a cellular radio communication system which supports antenna diversity and temporary transmission interruptions, and further relates to user and base stations for such a system.
In a radio communication system with antenna diversity in which two stations communicate with one another by radio, at least one of these stations is fitted with multiple antennas and is able to emit a radio signal for transmission to the other respective station via the individual antennas, weighted in each case with the coefficients of a weighting vector, so as to achieve a radiation pattern of the radio signal selectively adjusted to the other station. The information needed in order to adjust the weighting vector successfully is measured by the other station and delivered as back-coupling information to the first station.
A known system operating in this manner is the UMTS mobile radio communication system. This system has, in addition to open-loop operating states which do not need back-coupling information, two operating states with closed-loop antenna diversity, hereinafter designated closed-loop mode 1 and closed-loop mode 2. In an operating state without closed-loop antenna diversity, the user stations of the UMTS system generally do not carry out the measurements which are necessary in order to determine the back-coupling information since this is not required in an open-loop operating state or possibly may not even be measurable. If, however, a switch is made to a state with closed-loop antenna diversity, then from the outset a weighting vector must be available which the base station can use for transmitting to the user stations. Default weighting vectors are therefore predefined for the two operating states with closed-loop antenna diversity, which weighting vectors are used during the first time slot after transition to an operating state with antenna diversity and which, as the user stations deliver back-coupling information to the base station, are replaced by current weighting vectors.
The transition to an operating state with antenna diversity may relate both to the entire base station and all user stations connected to this base station and to an individual user station which is connected to a base station. The latter is the situation which occurs more frequently in practice.
After the transition to one of the states in closed-loop antenna diversity, the UMTS base station assumes that an uplink signal which it receives from a user station will contain back-coupling information via which it is able to select a current weighting vector which is well suited to the user station's actual reception conditions.
Temporary interruptions in the uplink and downlink signals may be necessary in mobile radio communication systems in order to enable a user station to carry out field strength measurements on radio signals of neighboring base stations and to check whether one of these neighboring base stations possibly delivers better reception conditions than the base station, designated here as the partner station, with which the user station is presently communicating.
In the case of a simple user station in which the transmitting and receiving sections cannot be tuned independently of one another, the transmitting section must be detuned during the measurements of the signal of the neighboring station so that interruptions here occur simultaneously in the downlink and the uplink respectively.
In such a case, the closed-loop operating state can be initialized after the end of the interruption in the same way as when switching to a state with closed-loop antenna diversity from another operating state.
In order to enable a user station with a single receiving section to carry out measurements on signals of neighboring base stations, an interruption of the downlink signal is unavoidable since the receiving section has to be tuned in order to carry out the measurements to the frequency of the neighboring base station and cannot in this period track the signal of the partner station. If the transmitting section of the user station can be tuned independently of the receiving section, then during the downlink interruption transmission can continue in the uplink.
In a user station with multiple receiving sections, one of which can remain constantly tuned to the downlink signal of the partner station and the second of which can be used for measurements of neighboring base stations, downlink interruptions are not necessary. However, temporary interruptions of the uplink signal may be necessary if transmission of the uplink signal would disrupt the measurements to be carried out; e.g., due to closely adjacent frequencies.
In order to make optimum use of the transmission capacity of a mobile radio communication system, it is of course desirable to limit as far as possible the extent of uplink or downlink interruptions which are associated with measurements of signals of neighboring stations and to allow interruptions independent of one another only in the uplink or only in the downlink. However, this raises a number of problems. Firstly, during an uplink interruption, the user station cannot deliver the back-coupling information which the base station needs in order to be able to continue to transmit the downlink signal to the user station. Secondly, the user station is unable during a downlink interruption to carry out the necessary measurements which are required in order to determine the back-coupling information.
In addition, consideration also must be given to the situation where a transition to an operating state with closed-loop antenna diversity falls within an interruption in the uplink and/or downlink transmission. Regardless of whether only the uplink, only the downlink or both are interrupted, the base station has from the start of the closed-loop operating state until the end of the interruption no current back-coupling information available to it with which it could control beam profiling.
For the reasons stated, with the conventional UMTS system, an optimum transition between operating states with and without closed-loop antenna diversity is possible only if the base station's communication with the user stations affected by the transition is not being interrupted. Ensuring this considerably complicates switching between operating states with and without closed-loop antenna diversity. If a base station serves a large number of user stations, then it is practically impossible to effect a change of operating state for all user stations simultaneously, since there usually will always be one among them whose communication is temporarily interrupted.
An object of the present invention is, therefore, to provide a method for operating a cellular radio communication system, a user station and a base station for such a system which support interruptions in the downlink and uplink signal independently of one another, and at any times, and which enable the system to switch between states with and without closed-loop antenna diversity in a simple way and regardless of possible transmission interruptions.